One class of prior art devices for generating and amplifying microwave radiation employs the interaction of a beam of electrons with an electromagnetic field inside an electron tube which is configured in a manner so as to provide for extraction of energy from rapidly streaming electrons in order to enhance the strength of an electromagnetic field. Such devices include klystrons, magnetrons, and travelling wave tubes, all capable of varied embodiments, but sharing the properties of a mechanism for accelerating the electrons into a stream which is initially uniform in velocity and direction, and a wave-supporting structure which allows for the resonant containment of an electromagnetic field. The interaction of the field with the beam of electrons allows the energy in the beam, which is initially in the form of direct current, to be transformed into alternating current at the frequency of the electromagnetic field. The energy in the electromagnetic field is then coupled out of the device and used for purposes as diverse as radar and microwave cooking. While there is no lower limit to the frequency at which such an oscillator can operate, there are high-frequency limitations imposed by losses of magnetic energy within the wave-supporting structure. The characteristic dimensions of the wave support structure are of the order of the wavelength of radiation to be generated. Surface roughness of the structure causes additional losses of electromagnetic energy in addition to losses due to interaction of the fields with the walls of the structure which increase as the square root of the frequency. The losses due to geometrical imperfections of fabrication grow more rapidly with frequency when the scale of the roughness is comparable to the wavelength of the radiation. To overcome these losses, higher electron densities are required within the region of interaction with the field, but these, in turn, are also limited by the natural tendency of charged particles to repel one another. Generation of power levels useful for communications by devices of this class is limited, in practice, to frequencies below approximately 100 GHz.
All klystrons currently operating may be referred to as operating in a longitudinal mode, in that their operation entails the bunching of electrons in a beam and the interaction of the bunched electrons with an electromagnetic field directed parallel to the motion of the electrons in the beam. Klystron devices of this sort are used to generate and to amplify microwave radiation.
In a klystron, the electron beam must interact in proper phase with the extracted electromagnetic field in order to amplify it. Operation of klystrons is thus sensitive to the time of flight between electron bunching and energy extraction, and thus to the voltage applied to accelerate the beam. This is true both in the linear and reflex configurations.